1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as a copy machine, a printer or a facsimile machine, and more particularly to a color image forming apparatus having an intermediate transfer belt.
2. Description of the Related Art
Generally, in an image forming apparatus designed to arrange respective image-forming units for four colors consisting of magenta (M), cyan (C), yellow (Y) and black (BK) along an intermediate transfer belt, so-called “tandem-type color image forming apparatus”, toner images of respective colors are sequentially transferred from the image-forming units to the intermediate transfer belt to form a color toner image on the intermediate transfer belt, and then color toner image is transferred to a recording sheet (hereinafter referred to simply as “sheet”). In order to correct color misregistration, this type of color image forming apparatus is designed to measure an amount of misregistration between patches of respective colors using a registration sensor and change an image write-start timing depending on the measured misregistration amount.
While the misregistration amount can be measured with a high degree of accuracy only if a linear speed of the intermediate transfer belt is accurately known, a linear speed of the intermediate transfer belt is varied depending on slip between the intermediate transfer belt and a drive belt, change in an outer diameter of a drive roller due to environmental variations and other factors. Thus, there is the need for measuring a linear speed of the intermediate transfer belt during the correction of color misregistration.
Heretofore, as measures for reducing color misregistration in an image, there has been a technique of forming markings on an intermediate transfer belt at given intervals and detecting the markings to obtain a speed of the intermediate transfer belt based on the detection result.
Further, Japanese Patent Laid-Open Publication No. 2003-233233 discloses a color image forming apparatus designed to detect a speed of a driven roller driven by an intermediate transfer belt, calculate a speed of the intermediate transfer belt based on the detected velocity of the driven roller, calculate a difference value between the calculated speed and a target speed of the intermediate transfer belt, and correct the speed of the intermediate transfer belt based on the calculated difference value. This Japanese Patent Laid-Open Publication No. 2003-233233 also discloses a technique of, during printing in one of a plurality of low-speed modes (½, ⅓, ¼ speed mode), performing a speed correction in a (1/1) speed mode, i.e., high speed mode, before forming an actual image on an intermediate transfer belt, and determining a speed of the intermediate transfer belt for each of the low-speed modes after completion of the speed correction so as to omit a speed correction for the intermediate transfer belt during subsequent printing.
In a color image forming apparatus designed to perform a print operation at high speed, even a slight speed variation in an intermediate transfer belt causes occurrence of color misregistration. If it is attempted to detect a linear speed of the intermediate transfer belt using markings as in the conventional technique, the markings have to be formed on the intermediate transfer belt using stickers or the like at several-micron-order intervals. It is technically difficult to form such markings.
If it is attempted to obtain a linear speed of an intermediate transfer belt based on a rotation speed of a driven roller as in the Japanese Patent Laid-Open Publication No. 2003-233233, a difference between the rotation speed of the driven roller and the linear speed of the intermediate transfer belt is likely to occur due to slip between the intermediate transfer belt and the driven roller to cause difficulty in accurately obtaining the linear speed of the intermediate transfer belt based on the rotation speed of the driven roller. This leads to a problem about difficulty in correcting color misregistration with a high degree of accuracy.